The overall objective of the proposed research is to contribute to our understanding of how intracellular Ca2+ regulates the excitability of hippocampal cells. Our interest in this issue is prompted by three recent developments. First, results were obtained showing that NMDA or glutamate, a presumed excitatory neurotransmitters in the hippocampus, caused sustained, significant increases of intracellular Ca2+ in isolated adult hippocampal cells. Secondly, it was discovered that the GABAA receptors in the hippocampal cells can no longer be activated by the ligand when intracellular Ca2+ increased beyond 5x10-6M. This latter finding is particularly interesting since previous data show that a blockade of inhibition mediated by GABAA receptors can led to epileptiform discharge in the hippocampus. Thirdly, we have succeeded in developing a recording system which allowed the changing of the intracellular contents during whole cell recording using isolated hippocampal cells. This technical advancement enables one to directly address questions regarding intracellular function of Ca2+. Proposed experiments will be carried out using acutely dissociated cells from the hippocampus of adult guinea-pigs. The specific objectives are: (1) to examine the role of intracellular Ca2+ in controlling the resting potential of the neurons. Experiments will characterize ionic currents activated by increases in Ca2+ when cells are voltage-clamped at their resting potential. The ionic basis and pharmacology of the currents will be studied. (2) To define the modulatory role of intracellular Ca2+ on voltage(V)-gated Ca2+ and K+ currents and (3) To compare the actions of elevated intracellular Ca2+ resulting from intracellular perfusion with that caused by entry through V- or ligand- gated channels. The study thus directly evaluates the control of cellular and perhaps circuitry (via GABAA receptor modulation) excitation by intracellular Ca2+. The results will contribute to our understanding of the generation and control of normal and abnormal activities in the hippocampus.